Precursors for chemical vapour deposition

ABSTRACT

Ti, Zr Hf and La precursors for use in MOCVD techniques have a ligand of the general formula OCR 1 (R 2 )CH 2 X, wherein R 1  is H or an alkyl group, R 2  is an optionally substituted alkyl group and X is selected from OR and NR 2 , wherein R is an alkyl group or a substituted alkyl group.

This invention concerns precursors for chemical vapour deposition. This invention is particularly, but not exclusively concerned with precursors for the growth of zirconium oxide (ZrO₂), hafnium oxide (HfO₂), zirconium oxide/silicon oxide (ZSO) and hafnium oxide/silicon oxide (HSO) by chemical vapour deposition.

Thin films of ZrO₂ and HfO₂ and the related silicates ZSO and HSO have important technological applications. In particular, they have high permittivities and are relatively stable in contact with silicon, making them the prime candidates to replace SiO₂ as gate dielectric layers in next-generation MOSFET devices in integrated Si circuits. Metalorganic chemical vapour deposition (MOCVD) is an attractive technique for the deposition of these materials, offering the potential for large area growth, good composition control and film uniformity, and excellent conformal step coverage at device dimensions less than 2 μm, which is particularly important in microelectronics applications.

An essential requirement for a successful MOCVD process is the availability of precursors with the appropriate physical properties for vapour phase transport and a suitable reactivity for deposition. There must be an adequate temperature window between evaporation and decomposition, and for most electronics applications oxide deposition is restricted to temperatures in the region of 500° C., to prevent degradation of the underlying silicon circuitry and metal interconnects.

There are a number of problems associated with existing Zr and Hf CVD precursors. For instance, the halides ZrCl₄ and HfCl₄ are low volatility solids which need substrate temperatures of 800° C. and above for oxide deposition. Metal β-diketonates, such as [Zr(thd)₄] (thd=2,2,6,6-tetramethylheptane-3,5-dionate) also require high substrate temperatures (>600° C.) for oxide growth. These are incompatible with the requirements of the electronics industry. Metal alkoxides are more attractive CVD precursors as they allow lower deposition temperatures. However, the majority of [Zr(OR)₄] and [Hf(OR)₄] complexes are dimeric or polymeric with limited volatility, due to the pronounced tendency of the Zr(IV) and Hf(IV) to expand their coordination sphere to six, seven or eight. In order to inhibit oligomerisation, sterically demanding ligands such as tert-butoxide have been employed, and [Zr(OBu^(t))₄] (D. C. Bradley, Chem. Rev. 1989, 89, 1317) and [Hf(OBu^(t))₄) (S. Pakswer & P Skoug, in “Thin dielectric oxide films made by oxygen assisted pyrolysis of alkoxides”, The Electrochem. Soc., Los Angeles, Calif., USA, 1970, 619-636) have been successfully used for the CVD of ZrO₂ and HfO₂. However, these mononuclear precursors contain unsaturated four-coordinate metal centres and the tert-butoxide ligand undergoes a catalytic decomposition reaction in the presence of trace water. This makes them highly air and moisture sensitive and susceptible to pre-reaction in the CVD reactor. Their reactivity also leads to a greatly reduced shelf life, especially in solution-based liquid injection CVD applications.

An object of this invention is to provide stable and volatile Ti, Zr and Hf precursors suitable for use in chemical vapour deposition techniques.

It has been surprisingly found that the donor functionalised alkoxy ligand 1-methoxy-2-methyl-2-propanolate [OCMe₂CH₂OMe, mmp] is effective in inhibiting oligomerisation in Zr and Hf alkoxide complexes, as well as increasing the ambient stability of the complexes.

Accordingly the present invention provides Ti, Zr, Hf and La precursors for use in MOCVD techniques having a ligand of the general formula OCR¹(R²)CH₂X, wherein R¹ is H or an alkyl group, R² is an optionally substituted alkyl group and X is selected from OR and NR₂, wherein R is an alkyl group or a substituted alkyl group.

Precursors according to a first preferred embodiment of the invention have the following general formula: M(L)_(x)[OCR¹(R²)CH₂X]_(4-x) wherein M is a metal selected from Ti, Zr and Hf, L is a ligand, x is a number from 0 to 3 and R¹, R² and X are as defined above.

The preferred ligand L is an alkoxy group having from 1 to 4 carbon atoms, of which tertiary-butoxide (OBu^(t)) group is most preferred, although other groups such as iso-propoxide (OPr^(i)) can be employed.

The preferred ligand of the formula OCR¹(R²)CH₂X is 1-methoxy-2-methyl-2-propanolate (mmp) but other donor functionalised alkoxide ligands may also carry out the desirable function of inhibiting oligomerisation in Zr, Hf and Ti alkoxides for use in the invention. These include but are not limited to OCH(Me)CH₂OMe, OCEt₂CH₂OMe, OCH(Bu^(t))CH₂OEt, OC(Bu^(t))₂CH₂OEt, OC(Pr^(i))₂CH₂OEt, OCH(Bu^(t))CH₂NEt₂, OC(Pr^(i))₂CH₂OC₂H₄OMe and OC(Bu^(t))(CH₂OPr^(i))₂.

The invention further provides a method of making Ti, Zr and Hf precursors for use in MOCVD techniques comprising reacting mmpH with the corresponding metal alkoxide or metal alkylamide in appropriate molar proportions.

The new alkoxide complexes Zr(OBu^(t))₂(mmp)₂, Zr(mmp)₄, Hf(OBu^(t))₂(mmp)₂, and Hf(mmp)₄ have been synthesised by the addition of mmpH to Zr(OBu^(t))₄ and Hf(OBu^(t))₄ in appropriate molar proportions. The complexes have high vapour pressures suitable for MOCVD, and are also much less reactive to air and moisture than Zr(OR)₄ compounds, wherein R is an alkyl group, making them easier to handle and use in MOCVD. The reduced air-sensitivity of these new Zr and Hf complexes arises from the replacement of the highly moisture sensitive tert-butoxide groups in [Zr(OBu^(t))₄] and [Hf(OBu^(t))₄] with the mmp ligand, which is much less susceptible to hydrolysis. The complexes are further stabilised to hydrolysis by an increase in the coordination number of the central Zr or Hf atom.

According to a second preferred embodiment the invention can be extended to other metals, which have large atomic radii and are highly positively charged, such as lanthanum, in which case preferred precursors have the following general formula La[OCR¹(R²)CH₂X]₃ wherein R¹ is H or an alkyl group, R² is an optionally substituted alkyl group and X is selected from OR and NR₂, wherein R is an alkyl group or a substituted alkyl group.

The preferred ligand for this preferred embodiment of the invention is 1-methoxy-2-methyl-2-propanolate [OCMe₂CH₂OMe], although other donor functionalised alkoxide ligands may be used. These may include but are not limited to OCH(Me)CH₂OMe, OCEt₂CH₂OMe, OCH(Bu^(t))CH₂OEt, OC(Bu^(t))₂CH₂OEt, OC(Pr^(i))₂CH₂OEt, OCH(Bu^(t))CH₂NEt₂, OC(Pr^(i))₂CH₂OC₂H₄OMe and OC(Bu^(t))(CH₂OPr^(i))₂.

The invention also provides a preferred method of making precursors according to the second preferred embodiment comprising reacting mmpH with La{N(SiMe₃)₂}₃ in appropriate molar proportions.

Precursors according to the invention may be used in depositing single or mixed oxide layers or films by conventional MOCVD, in which the precursor is contained in a metalorganic bubbler, or by liquid injection MOCVD, in which the precursor is dissolved in an appropriate inert organic solvent and then evaporated into the vapour phase using a heated evaporator. The precursors may also be suitable for use in the deposition of zirconium oxide, hafnium oxide and titanium oxide films by other chemical vapour deposition techniques, such as atomic layer deposition (ALD).

The precursors can be used for the MOCVD of ZrO₂, HfO₂, and TiO₂, La₂O₃, and in combination with other precursors for the MOCVD of complex oxides containing zirconium oxide, hafnium oxide, and lanthanum oxide such as ZSO and HSO and La-silicate.

The precursors can also be used in combination for the MOCVD of complex oxides. Examples include the MOCVD of bismuth-titanate from the combinations Bi(mmp)₃/Ti(OPr^(i))₂(mmp)₂ or Bi(mmp)₃/Ti(mmp)₄.

The invention will now be further described with reference to the accompanying drawings, in which:

FIG. 1 shows an envisaged structure for M(OBu^(t))₂(mmp)₂ (M=Zr or Hf);

FIG. 2 shows the molecular structure of Hf(mmp)₄. Zr(mmp)₄ has a similar structure; and

FIG. 3 shows laser Raman spectra ZrO₂ and HfO₂ films grown by liquid injection MOCVD using Zr(OBu^(t))₂(mmp)₂ or Hf(OBu^(t))₂(mmp)₂.

The invention will now be further described by means of the following Examples.

EXAMPLE 1 Preparation of Zr(OBu^(t))₂(mmp)₂

2.8 ml (2.69 g, 7.0 mmol) Zr(OBu^(t))₄ was dissolved in hexane (ca. 40 ml). mmph (1.6 ml, 1.44 g, 13.9 mmol) was added dropwise, the mixture was heated to reflux and stirring continued for a further 2 hours. The solution was cooled to room temperature and the volatiles removed by evaporation under reduced pressure. The product was recrystallised from hexane to give a white crystalline solid.

M.pt.: 96-101° C. (uncorrected) Microanalysis: Calc. C, 48.71; H, 9.10. Found: C, 46.32; H, 8.77% ¹H NMR: (400 MHz, d₈-tol) 1.19 (s, 12H, OC(CH ³ )₂CH₂OCH₃), 1.37 (s, 18H, OC(CH ³ )₃), 3.23 (s, 4H, OC(CH₃)₂CH ² OCH₃), 3.40 (s, 6H, OC(CH₃)₂CH₂OCH ³ ). ¹³C NMR: 34.1 (OC(CH ³ )₂CH₂OCH₃), 38.5 (OC(CH ³ )₃), 65.4 (OC(CH₃)₂CH₂OCH ³ ), 78.6 (OC(CH₃)₂CH₂OCH₃ and OC(CH₃)₃), 90.5 (OC(CH₃) ² CH ² OCH₃). IR: (ν cm⁻¹, Nujol, NaCl) 3588(w), 3442(w), 2725(m), 2360(w), 1356(s), 1277(m), 1227(m), 1206(s), 1177(s), 1115(s), 1080(s), 1012(s), 974(s), 936(s), 801(s), 782(s), 595(s).

An envisaged structure for Zr(OBu^(t))₂(mmp)₂ is shown in FIG. 1 of the drawings.

EXAMPLE 2 Preparation of Zr(mmp)₄

2.0 g (5.2 mmol) Zr(OPr^(i))₄.Pr^(i)OH was dissolved in hexane (ca. 40 ml). mmpH (2.6 ml, 2.35 g, 22.5 mmol) was added dropwise, the mixture heated to reflux and stirring was continued for 2 hours. The mixture cooled to room temperature and the volatiles was removed by evaporation under reduced pressure to give the product as a white viscous oil. (Yield: 2.4 g, 94%).

Zr(mmp)₄ can also be synthesised from the corresponding zirconium alkylamide complexes, Zr(NR₂)₄. For instance, by the dropwise addition of mmpH (6.9 g, 65.8 mmol) to a stirred solution of [Zr(NEt₂)₄] ( 5.0 g, 13.2 mmol) in hexane (50 cm³). The mixture was boiled under reflux for 2 hr. and then allowed to cool to room temperature. Volatiles were removed in vacuo to give the product (yield 6.25 g, 94%).

Microanalysis: Calc. C, 47.67; H, 8.82 Found: C,47.80; H, 8.79%. ¹H NMR: (400 MHz, d₈-tol): 1.21 (s, OC(CH ³ )₂CH₂OCH₃), 3.16 (s, OC(CH₃)₂CH ² OCH₃), 3.27 (s, OC(CH₃)₂CH₂OCH ³ ) ¹³C NMR: (100 MHz, d₈-tol): 32.1 (OC(CH ³ )₂CH₂OCH₃), 64.8 (OC(CH₃)₂CH₂OCH ³ ), 76.0 (OC(CH₃)₂CH₂OCH₃), 88.5 (OC(CH₃) ² CH₂OCH₃). IR: (ν cm⁻¹, Nujol, NaCl) 3589(w), 3448(w,br), 2724(m), 2346(w), 1377(s), 1322(m), 1279(m), 1239(m), 1176(s), 1134(m), 1114(s), 1081(m), 1018(s), 996(m), 982(s), 958(m), 937(m), 917(m), 845(m), 804(m), 784(m), 594(s).

EXAMPLE 3 Preparation of Hf(OBu^(t))₂(mmp)₂

3.5 ml (4.0 g, 8.5 mmol) Hf(OBu^(t))₄ was dissolved in hexane (ca. 40 ml) to give a yellow solution. MmpH (2.0 ml, 1.79 g, 19.0 mmol) was added dropwise, the mixture heated to reflux and stirring continued for 2 hours. The solution was allowed to cool and the volatiles removed by boiling under reduced pressure. The crude product was recrystallised from hexane to give a white crystalline solid.

(Yield: 4.4 g, 97%). M. Pt: 100-104° C. (uncorrected) Microanalysis: Calc. C, 40.71; H, 7.61. Found. C, 38.93; H, 7.30% ¹H NMR: (400 MHz, d₈-tol): δ=1.18 (s, 12H, OC(CH ³ )₂CH₂OCH₃), 1.38 (s, 18H, OC(CH ³ )₃), 3.21 (s, 12H, OC(CH₃)₂CH ² OCH₃), 3.42 (s, 12H, OC(CH₃)₂CH₂OCH ³ ) ¹³C NMR: (100 MHz d₈-tol): δ=34.4 (OC(CH ³ )₂CH₂OCH₃), 38.6 (OC(CH ³ )₃), 65.7, (OC(CH₃)₂CH₂OCH ³ ), 78.0, 79.1 (OC(CH₃)₂CH₂OCH₃ and OC(CH₃)₃), 90.9 (OC(CH₃)₂ CH ² OCH₃), IR: (ν cm⁻¹, Nujol, NaCl): 3441 (w), 2726(m), 2256(w), 1272(s), 1177(s), 1074(s), 1016(s), 976(s), 802(s), 782(s), 593(s).

An envisaged structure for Hf(OBu^(t))₂(mmp)₂ is shown in FIG. 1 of the drawings.

EXAMPLE 4 Preparation of Hf(mmp)₄

4.0 ml (5.56 g, 11.9 mmol) [Hf(NEt₂)₄] was dissolved in hexane (60 ml). Hmmp (7.0 ml, 6.3 g, 60 mmol) was added dropwise and the mixture refluxed for 90 mins. Volatiles were removed in vacuo to give the product as a yellow viscous oil.

(Yield: 6.88 g, 97.5 %). Microanalysis: Calc. C, 40.63; H, 7.52. Found. C, 39.85; H, 7.32% ¹H NMR: 1.30 (s, 24H, OC(CH ³ )₂CH₂OCH₃), 3.28 (s, 8H, OC(CH₃)₂CH ² OCH—₃), 3.36 (s, 12H, OC(CH₃)₂CH₂OCH ³ ) ¹³C NMR: 34.74 (OC(CH₃)₂CH₂OCH₃), 65.16 (OC(CH₃)₂CH₂OCH₃), 79.83 (OC(CH₃)₂CH₂OCH₃), 90.25 (OC(CH₃)₂ CH₂OCH₃) IR: (Nujol/NaCl): 3585(w), 3450(w,br), 2722(m), 1366(s), 1356(vs), 1268(s), 1242(s), 1214(vs), 1177(vs), 1115(vs), 1079(vs), 1045(vs), 1026(vs), 996(vs), 975(vs), 936(vs), 912(m), 802(s), 779(s), 594(vs).

An envisaged structure for Hf(mmp)₄ is shown in FIG. 2 of the accompanying drawings.

EXAMPLE 5 Preparation of Zr(OPr^(i))₂(mmp)₂

1.06 g (2.75 mmol) Zr(OPr^(i))₄.Pr^(i)OH was dissolved in hexane (ca 40 ml). 1-methoxy-2-methyl-2-propanol [mmpH] (0.65 ml, 0.57 g, 5.5 mmol) was added dropwise, the mixture was heated to reflux and stirring continued for a further 2 hours. The solution was cooled to room temperature and the volatiles removed by evaporation under reduced pressure. The product was isolated as a white viscous oil.

Microanalysis: Calc. C, 46.23; H, 8.73. Found: C, 44.17; H, 8.47; ¹H NMR (400 MHz, d₈-tol): 1.26 (s, OC(CH ³ )₂CH₂OCH₃), 1.32 (d, OCH(GH ³ )₂), 3.26 (2, OC(CH₃)₂CH ² OCH₃), 3.36 (s, OC(CH₃)₂CH₂OCH ³ ), 4.46 (m, OCH(CH₃)₂). ¹³C NMR (100 MHz, d₈-tol): 32.1 (OC(CH ³ )₂CH₂OCH₃), 34.2 (OCH(CH ³ )₂), 64.9 (OC(CH₃)₂CH₂OCH ³ ), 76.1, 76.4 (OCH(CH₃)₂ and OC(CH₃)₂CH₂OCH₃), 88.6 (OC(CH₃)₂ CH ² OCH₃). IR: (ν cm⁻¹, Nujol, NaCl) 3589(w), 3423(w), 2724(w), 2282(w), 1239(w), 1175(m), 1115(m), 1019(m), 959(m).

EXAMPLE 6 Preparation of Ti(OPr^(i))₂(mmp)₂

MmpH (2.81 g, 27 mmol) was added dropwise to 0.0135 moles) to a stirred solution of Ti(OPr^(i))₄ (3.84 g, 13.5 mmol) in hexane (20 ml). The mixture was boiled under reflux for 1½ hours and was then allowed to cool. The solvent was then removed in vacuo to give Ti(OPr^(i))₂(mmp)₂ as a colourless oil.

Microanalysis for TiC₁₆H₃₆O₄: (calculated) C,% 51.61; H,% 9.75; (experimental) C,% 51.20; H,% 9.92. ¹H NMR (C₆D₅CD₃ at 30° C.) δ 1.1 (26H, d, (CH ₃)₂CH; CH₃OCH₂(CH ₃)₂C); δ 3.2 (10H, two singlets, CH₃OCH ₂(CH₃)₂C ); δ 4.5,(2H, m, (CH₃)₂CH). ¹³C{¹H} NMR (C₆D₅CD₃, 30° C.): 32 (OC(CH₃)₂CH₂OCH₃), 33.4 (OCH(CH ³ )₂), 64.4 (OC(CH₃)₂CH₂OCH ³ ), 81.7 (OC(CH₃)₂CH₂OCH₃ 86.5 (OCH(CH₃)₂), 88 (OC(CH₃)₂ CH ₂OCH₃). IR (Nujol, cm⁻¹) 2972s, 2928s, 2869s, 2625w, 1463m, 1376m, 1360s, 1331m, 1277m, 1126s, 1001s, 850s, 778m., 629s.

EXAMPLE 7 Preparation of Ti(mmp)₄

MmpH (4.41 g, 42 mmol) was added dropwise to a stirred solution of Ti(NEt₂)₄ (2.85 g; 3 ml; 8.47 mmole) in hexane (20 ml) resulting in pale brown solution. The mixture was boiled under reflux for 1½ hours, allowed to cool, and then volatiles were removed in vacuo to give Ti (mmp)₄ as a pale brown oil.

Microanalysis for TiC₂₀H₄₄O₈: (calculated) C,% 52.17; H,% 9.63; (experimental) C,% 51.95; H,% 9.97. ¹H NMR (C₆D₅CD₃ at 30° C.) δ 1.3 (24H, s, CH₃OCH₂(CH ₃)₂C); δ 3.2 (20H, two singlets, CH₃OCH ² (CH₃)₂C). VT ¹H NMR showed sharp distinct peaks from −50 to +50 ° C.—no broadening was apparent. ¹³C{¹H} NMR (C₆D₅CD₃, 30° C.): 31.9 (OC(CH ³ )₂CH₂OCH₃), 64.5 (OC(CH₃)₂CH₂OCH₃), 81.7 (OC(CH₃)₂CH₂OCH₃), 87 (OC(CH₃)₂ CH₂OCH₃). IR (Nujol, cm⁻¹) 2975s, 2931s, 2876s, 2829m, 2625w, 1461m, 1360s, 1331m, 1277m, 12406m, 1116s, 1004s, 850m., 796s, 775s, 625s.

EXAMPLE 8 Preparation of La(mmp)₃

[La{N(SiMe₃)₂}₃] (2.89 g, 4.6 mmol) was dissolved in toluene (50 ml) and mmpH (2.2 ml, 1.96 g, 18.7 mmol) added dropwise under stirring. Stirring was continued at room temperature for a further 21 hours and the volatiles removed in vacuo to give the product as a brown viscous oil (Yield=1.8 g 87% with respect to La)

Microanalysis for LaC₁₅H₃₃O₆(calculated) C, % 40.18; H, % 7.43; (experimental) C, % 40.01; H % 7.38

EXAMPLE 9 Zirconium Oxide and Hafnium Oxide Deposition From Zr(OBu^(t))₂(mmp)₂, Zr(mmp)₄, Hf(OBu^(t))₂(mmp)₂ and Hf(mmp)₄

All four complexes were found to be excellent precursors for the deposition of ZrO₂ and HfO₂ thin films by MOCVD. The ZrO₂ and HfO₂ films were deposited by liquid injection MOCVD using the same general conditions shown in Table 1 below. TABLE 1 Growth conditions used for the growth of ZrO₂ or HfO₂ thin films by liquid injection MOCVD using Zr(OBu^(t))₂(mmp)₂, Zr(mmp)₄, Hf(OBu^(t))₂(mmp)₄ or Hf(mmp)₄ Substrate temperature 350-650° C. Reactor pressure 20-30 mbar Precursor solution concentration 0.1M in toluene Precursor solution injection rate 4-8 cm³ hr⁻¹ Evaporator temperature 130-150° C. Argon carrier gas flow rate 400-600 cm³ min⁻¹ Oxygen flow rate 100-150 cm³ min⁻¹ Substrates Si(100) Oxide growth rate 0.35-0.50 μm hr⁻¹

The identity of the films was confirmed as ZrO₂ or HfO₂ by laser Raman spectroscopy (see FIG. 3). Raman spectra of ZrO₂ and HfO₂ films grown from Zr(OBu^(t))₂(mmp)₂ or Hf(OBu^(t))₂(mmp)₂ are shown in FIG. 3. Comparison with bulk crystalline data showed that these films were predominantly in the α- or monoclinic phase. 

1. A precursor composition comprising a material selected from the group consisting of Ti, Zr, Hf, and La compounds, and a ligand of the formula OCR′(R²)CH₂X, wherein R¹ is H or an alkyl group, R² is an optionally substituted alkyl group and X is selected from OR and NR₂, wherein R is an alkyl group or a substituted alkyl group.
 2. A precursor for use in MOCVD techniques having the following general formula: M(L)_(x)[OCR¹(R²)CH₂X]_(4-x) wherein M is a metal selected from Ti, Zr and Hf, l is a ligand, x is a number from 0 to 3, R¹ is H or an alkyl group, R² is an optionally substituted alkyl group, and X is selected from OR and NR₂, wherein R is an alkyl group or a substituted alkyl group.
 3. A precursor as claimed in claim 2, wherein the ligand L is an alkoxy group having from 1 to 4 carbon atoms.
 4. A precursor as claimed in claim 3, wherein the ligand L is selected from tertiary-butoxide (OBu^(t)) and iso-propoxide (OPr^(i)).
 5. A precursor as claimed in claim 2, wherein the ligand of the formula OCR¹(R²)CH2X is 1-methoxy-2-methyl-2-propanolate (mmp).
 6. A precursor as claimed in claim 2, wherein the ligand of the formula OCR¹(R²)CH₂X is selected from OCH(Me)CH₂OMe, OCEt₂CH₂OMe, OCH(Bu^(t))CH₂OEt, OC(Bu^(t))₂CH₂OEt, OC(Pr^(i))₂CH₂OEt, OCH(Bu^(t))CH₂NEt₂, OC(Pr^(i))₂CH₂OC₂H₄OMe and OC(Bu^(t))(CH₂Opr^(i))₂.
 7. A precursor according to claim 2, wherein said precursor is Zr(OBu^(t))₂(mmp)₂.
 8. A precursor according to claim 2, wherein said precursor is Zr(mmp)₄.
 9. A precursor according to claim 2, wherein said precursor is Hf(OBu^(t))₂(mmp)₂.
 10. A precursor according to claim 2, wherein said precursor is Hf(mmp)₄
 11. A method of making Ti, Zr and Hf precursors for use in MOCVD techniques comprising reacting HOCR¹(R²)CH₂X with the corresponding metal alkoxide or metal alkylamide in appropriate molar proportions.
 12. A precursor for use in MOCVD techniques having the following general formula: La[OCR¹(R²)CH₂X]₃ wherein R¹ is H or an alkyl group, R² is an optionally substituted alkyl group and X is selected from OR and NR₂, wherein R is an alkyl group or a substituted alkyl group.
 13. A precursor as claimed in claim 12, wherein the ligand is 1-methoxy-2methyl-2-propanolate (OCMe₂CH₂OMe).
 14. A precursor as claimed in claim 13, wherein the ligand is selected from OCH(Me)CH₂OMe, OCEt₂CH₂OMe, OCH(Bu^(t))CH₂OEt, OC(Bu^(t))₂CH₂OEt, OC(Pr^(i))₂CH₂OEt, OCH(Bu^(t))CH₂NEt₂, OC(Pr^(i))₂CH₂OC₂H₄OMe and OC(Bu^(t))(CH₂OPr^(i))₂.
 15. A precursor according to claim 12, wherein said precursor is La(mmp)₃.
 16. A method of making precursors as claimed in claim 12 comprising reacting HOCR¹(R²)CH₂X with La{N(SiMe₃)₂}₃ in appropriate molar proportions.
 17. A method of depositing single or mixed oxide layers or films by conventional MOCVD, in which the precursor is contained in a metalorganic bubbler, or by liquid injection MOCVD, in which the precursor is dissolved in an appropriate inert organic solvent and then evaporated into the vapour phase using a heated evaporator, wherein at least one of the precursors is as defined in claim
 1. 18. A precursor as claimed in claim 12, wherein the ligand L is an alkoxy group having from 1 to 4 carbon atoms.
 19. A precursor as claimed in claim 12, wherein the ligand L is selected from tertiary-butoxide (OBu^(t)) and iso-propoxide (OPr^(i)).
 20. A method of providing a ZrO₂, ZrSO, HfO₂, HfSO, TiO₂, or TiSO film on a silicon substrate, the method comprising using metalorganic chemical vapor deposition to deposit the metal oxide, wherein the deposition utilizes a precursor material of the formula: M(L)_(x)[OCR¹(R²)CH₂X]_(4-x) wherein M is a metal selected from Ti, Zr and Hf, L is a ligand, x is a number from 0 to 3, R¹ is H or an alkyl group, R² is an optionally substituted alkyl group, and X is selected from OR and NR₂, wherein R is an alkyl group or a substituted alkyl group.
 21. A method of providing a LaO₂ or LaSO film on a silicon substrate, the method comprising using metalorganic chemical vapor deposition to deposit the metal oxide, wherein the deposition utilizes a precursor material of the formula: La[OCR¹(R²)CH₂X]₃ wherein R¹ is H or an alkyl group, R² is an optionally substituted alkyl group and X is selected from OR and NR₂, wherein R is an alkyl group or a substituted alkyl group. 